1. Field of the Invention
The present invention relates to a method for manufacturing positive electrodes for alkaline storage batteries.
2. Description of Related Art
In recent years, alkaline storage batteries have attracted wide attention as a power source for portable equipment and those of electric cars or hybrid electric vehicles. Accordingly, the demand for higher performance of the alkaline storage batteries has been intensified. In particular, nickel metal-hydride secondary batteries provided with positive electrodes using nickel hydroxide and negative electrodes using hydrogen absorbing alloy rapidly have become widespread as a secondary battery with a high energy density and excellent reliability.
As the positive electrodes for the alkaline storage batteries, electrode plates obtained by filling a porous metal sheet with an active material for the positive electrode that contains nickel hydroxide particles as the main component are used. The thickness of these electrode plates can be adjusted by pressing with a roller press after filling the porous metal sheet with a paste of the active material.
In the positive electrodes for the alkaline storage batteries, it is effective that pores in the porous metal sheet, in which the active material is filled, are substantially spherical, in terms of extending lifetime and improving filling characteristics of the active material. However, the conventional positive electrodes described above have had a problem that the oblateness of the pores in the porous metal sheet increases when being pressed with the roller press.
With the foregoing in mind, it is an object of the present invention to provide a method for manufacturing a positive electrode for an alkaline storage battery that achieves excellent filling characteristics for an active material for the positive electrode and longer lifetime of the battery.
In order to achieve the above-mentioned object, a method for manufacturing a positive electrode for an alkaline storage battery of the present invention includes a first process of filling a paste of an active material for the positive electrode in a first porous metal sheet provided with a plurality of oblate pores whose major axes arranged substantially in one direction, and a second process of pressing the first porous metal sheet that has undergone the first process using a roller press such that the one direction and a direction of a rotation axis of a roller in the roller press substantially are parallel, so as to produce a positive electrode sheet provided with a second porous metal sheet. In the second porous metal sheet, a value obtained by dividing a length of the pores in the one direction by that of the pores in a direction perpendicular to the one direction and parallel to a surface of the second porous metal sheet averages in the range of 0.9 to 1.1. In the above manufacturing method, a ratio of length and breadth of the pores in the porous metal sheet contained in the positive electrode is 1 or close to 1. Thus, in the alkaline storage battery using the positive electrode that is manufactured by the above manufacturing method, a reaction during charging and discharging occurs in a substantially uniform manner, achieving a longer lifetime. Also, the filling characteristics of the active material improve in the positive electrode that is manufactured by the above manufacturing method.
The above manufacturing method further may include a preparatory process of producing the first porous metal sheet by pressing a porous metal sheet provided with a plurality of pores using a roller press before the first process. According to the above embodiment, the first porous metal sheets provided with the pores of various shapes can be formed by changing a pressing condition in the preparatory process and the porous metal sheet to be used. Therefore, this embodiment reduces the restriction of the conditions in the first and second processes.
In the above manufacturing method, in the second process, the first porous metal sheet may pass through the roller press after being rotated substantially by 90xc2x0 around an axis perpendicular to a principal plane of the first porous metal sheet from a direction in which the porous metal sheet has passed through the roller press in the preparatory process. This embodiment makes it easier to implement the manufacturing method of the present invention. In the present invention, xe2x80x9cbeing rotated substantially by 90xc2x0xe2x80x9d means that longitudinal and transverse directions of the sheet are exchanged with each other.
In the above manufacturing method, the porous metal sheet may be formed of nickel.
In the above manufacturing method, the paste of the active material may contain nickel hydroxide.
The pressing process in the preparatory process and the second process may be carried out one time or a plurality of times.